Detectors of the subject type find wide application for measuring neutron, gamma and other forms of radiation. Gaseous HE-3, for example, has been widely employed in detection and spectroscopy of neutron radiation. The neutron-induced reaction EQU .sub.2 He.sup.3 +.sub.0 n.sup.1 .fwdarw..sub.1 H.sup.1 +.sub.1 H.sup.3( 1)
has a positive Q-value of 765 keV, resulting in production of an energetic proton and triton for each interacting neutron.
Typical detectors comprise an ionization or proportional counter filled with He-3 gas. To enhance efficiency, such detectors are operated at pressures of several atmospheres to increase the density of helium target nuclei. Maximum pressure, however, is limited to about ten atmospheres because of physical and mechanical problems. For example, construction of seals and electrical feedthroughs becomes difficult and unreliable at higher pressures. Moreover, high gas pressures slow ion and electron migration times, and therefore decrease timing precision. Conventional detectors have timing resolutions typically on the order of hundreds of nanoseconds, which is insufficient for applications which require timing precision.
Another problem with conventional He-3 detectors is the so-called "wall effect". At pressures obtainable with conventional designs, the travel distance or range of reaction products is not small compared with the physical dimensions of the detectors (typically greater than or equal to about 5 cm). Therefore, a significant fraction of events results in only partial energy loss within the detector gas, and a corresponding reduction in observed output pulse amplitude. A heavier gas component, such as argon or krypton, may be added to reduce the range of reactor products, but at the expense of detection efficiency.
It has been proposed to intermix a powdered radiation detector with a separate solid phase scintillation medium, such as a scintillating polymer. Equation (1) is an example of a gaseous detection medium. Other detection materials having lithium and boron target nuclei exhibit the following reactions with thermal neutrons: EQU .sub.5 B.sup.10 +.sub.0 n.sup.1 .fwdarw..sub.3 Li.sup.7 +.sub.2.alpha..sup.4( 2) EQU .sub.3 Li.sup.6 +.sub.0 n.sup.1 .fwdarw..sub.1 H.sup.3 +.sub.2.alpha..sup.4( 3)
The thermal neutron cross section for boron is 3800 barns with a Q-value of 2.3 MeV, and that of lithium is 1000 barns at a Q-value of 4.78 MeV. The more energetic alpha particle of the boron reaction and triton from the lithium reaction are the principal sources of light produced in the scintillation medium. However, the proposed intermixtures do not exhibit desired efficiency. Specifically, failure of the index of refraction of the detection powder to approximate or match that of the scintillation medium increases light scatter, thereby increasing light absorption and reducing output efficiency of the scintillation medium. Doping of the scintillation plastic, whereby the target nuclei are chemically bonded to the plastic, generally degrades scintillation efficiency.